The present invention relates to networks and particularly to networks of computers that communicate data and other information.
Wide Area Networks
With the increased bandwidth available through transmission channels, for example increases from T1 to T3, and with the increase in bandwidth provided by broadband services such as SONET, larger enterprises are evaluating new applications which require higher speed communications. These new applications will dramatically enhance business productivity, but will require vastly improved network control and management facilities. However, neither private networks nor common carriers have fully addressed the emerging needs of the new communication environment.
Computer Networks
In the computer field, in order for users to have access to more information and to greater resources than those available on a single computer, computers are connected through networks.
In a computer network, computers are separated by distance where the magnitude of the distance has a significant bearing on the nature of communication between computers. The distance can be short, for example, within the same computer housing (internal bus), can be somewhat longer, for example, extending outside the computer housing but within several meters (external bus), can be local, for example, within several hundred meters (local area networks, LANs), within tens of miles (metropolitan area networks, MANs) or can be over long distances, for example, among different cities or different continents (wide area networks, WANs).
Multi-Layer Communication Architecture
For networks, the communication facilities are viewed as a group of layers, where each layer in the group is adapted to interface with one or more adjacent layers in the group. Each layer is responsible for some aspect of the intended communication. The number of layers and the functions of the layers differ from network to network. Each layer offers services to the adjacent layers while isolating those adjacent layers from the details of implementing those services. An interlayer interface exists between each pair of adjacent layers. The interlayer interface defines which operations and services a layer offers to the adjacent layer. Each layer performs a collection of well-defined functions.
Many multi-layered communication architectures exist including Digital Equipment's Digital Network Architecture (DNA), IBM's System Network Architecture (SNA) and the International Standards Organization (ISO) Open System Interface (OSI).
The ISO architecture is representative of multi-level architectures and consists of a 7-layer OSI model having a physical link layer, a data link layer, a network layer, a transport layer, a session layer, a presentation layer, and an application layer.
In the OSI model, the physical layer is for standardizing network connectors and the electrical properties required to transmit binary 1's and 0's as a bit stream. The data link layer breaks the raw bit stream into discrete units and exchanges these units using a data link protocol. The network layer performs routing. The transport layer provides reliable, end-to-end connections to the higher layers. The session layer enhances the transport layer by adding facilities to help recover from crashes and other problems. The presentation layer standardizes the way data structures are described and represented. The application layer includes protocol handling needed for file transfer, electronic mail, virtual terminal, network management and other applications.
In the n-layer multi-layer models, layers 1, 2, . . . , n are assumed to exist in each host computer. Layers 1, 2, . . . , n in one host computer appear to communicate with peer layers 1, 2, . . . , n, respectively, in another host computer. Specifically, layer 1 appears to communicate with layer 1, layer 2 appears to communicate with layer 2 and so on with layer n appearing to communicate with layer n. The rules and conventions used in communications between the peer layers are collectively known as the peer level protocols. Each layer executes processes unique to that layer and the peer processes in one layer on one computer station appear to communicate with corresponding peer processes in the same layer of another computer station using the peer protocol.
Although peer layers appear to communicate directly, typically, no data is directly transferred from layer n on one computer station to layer n on another computer station. Instead, each layer n passes data and control information to the n-1 layer immediately below it in the same computer station, until the lowest layer in that computer is reached. The physical medium through which actual communication occurs from one computer station to another exists below the top layer n and typically below the bottom layer 1.
In order to provide communication to the top layer n of an n-layer network, a message, M, is produced by a process running in a top layer n of a source computer station. The message is passed from layer n to layer n-1 according to the definition of the layer n/n-1 interface. In one example where n equals 7, layer 6 transforms the message (for example, by text compression), and then passes the new message, M, to the n-2 layer 5 across the layer 5/6 interface. Layer 5, in the 7 layer example, does not modify the message but simply regulates the direction of flow (that is, prevents an incoming message from being handed to layer 6 while layer 6 is busy handing a series of outgoing messages to layer 5).
In many networks, there is no limit to the size of messages accepted by layer 4, but there is a limit imposed by layer 3. Consequently, layer 4 must break up the incoming messages into smaller units, prefixing a header to each unit. The header includes control information, such as sequence numbers, to allow layer 4 on the destination computer to put the pieces back together in the right order if the lower layers do not maintain sequence. In many layers, headers also contain sizes, times and other control fields.
Layer 3 decides which of the outgoing lines to use, attaches its own headers, and passes the data to layer 2. Layer 2 adds not only a header to each piece, but also a trailer, and gives the resulting unit to layer 1 for physical transmission. At the destination computer, the message moves upward, from lower layer 1 to the upper layers, with headers being stripped off as it progresses. None of the headers for layers below n are passed up to layer n.
Virtual Peer To Peer Communication
An important distinction exists between the virtual and actual communication and between protocols and interfaces. The peer processes in source layer 4 and the destination layer 4, for example, interpret their layer 4 communication as being "direct" using the layer 4 protocol without recognition that the actual communication transcends down source layers 3, 2, 1 across the physical medium and thereafter up destination layers 1, 2, and 3 before arriving at destination layer 4.
The virtual peer process abstraction assumes a model in which each computer station retains control over its domain and its communication facilities within that domain.
Communication Networks Generally
For more than a century, the primary international communication system has been the telephone system originally designed for analog voice transmission. The telephone system (the public switched network) is a circuit switching network because a physical connection is reserved all the way from end to end throughout the duration of a call over the network. The telephone system originally sent all its control information in the 4 kHz voice channel using in-band signaling.
To eliminate problems caused by in-band signaling, in 1976 AT&T installed a packet switching network separate from the main public switched network. This network, called Common Channel Interoffice Signaling (CCIS), runs at 2.4 kbps and was designed to move the signaling traffic out-of-band. With CCIS, when an end office needed to set up a call, it chose a channel on an outgoing trunk of the public switched network. Then it sent a packet on the CCIS network to the next switching office along the chosen route telling which channel had been allocated. The next switching office acting as a CCIS node then chose the next outgoing trunk channel, and reported it on the CCIS network. Thus, the management of the analog connections was done on a separate packet switched network to which the users had no access.
The current telephone system has three distinct components, namely, the analog public switched network primarily for voice, CCIS for controlling the voice network, and packet switching networks for data.
Future Communication Networks-ISDN
User demands for improved communication services have led to an international undertaking to replace a major portion of the worldwide telephone system with an advanced digital system by the early part of the twenty-first century. This new system, called ISDN (Integrated Services Digital Network), has as its primary goal the integration of voice and nonvoice services.
The investment in the current telephone system is so great that ISDN can only be phased in over a period of decades and will necessarily coexist with the present analog system for many years and may be obsolete before completed.
In terms of the OSI model, ISDN will provide a physical layer onto which layers 2 through 7 of the OSI model can be built.
Telephone Network Domains
In a telephone network, the system architecture from the perspective of the telephone network is viewed predominantly as a single domain. When communication between two or more callers (whether people or computers) is to occur, the telephone network operates as a single physical layer domain.
Communication Network Architectures
Most wide area networks have a collection of end-users communicating via a subnet where the subnet may utilize multiple point-to-point lines between its nodes or a single common broadcast channel.
In point-to-point channels, the network contains numerous cables or leased telephone lines, each one connecting a pair of nodes. If two nodes that do not share a cable are to communicate, they do so indirectly via other nodes. When a message (packet), is sent from one node to another via one or more intermediate nodes, the packet is received at each intermediate node in its entirety, stored there until the required output line is free, and then forwarded. In broadcast channels, a single communication channel is shared by all the computer stations on the network. Packets sent by any computer station are received by all the others. An address field within the packet specifies the intended one or more computer stations. Upon receiving a packet, a computer station checks the address field and if the packet is intended only for some other computer station, it is ignored.
Most local area networks use connectionless protocols using shared medium where, for example, all destination and source information is included in each packet and every packet is routed autonomously with no prior knowledge of the connection required.
In the above-identified application CONCURRENT MULTI-CHANNEL SEGMENTATION AND REASSEMBLY PROCESSORS FOR ASYNCHRONOUS TRANSFER MODE (ATM) an apparatus for concurrently processing packets in an asynchronous transfer mode (ATM) network is described. Packets that are to be transmitted are segmented into a plurality of cells, concurrently for a plurality of channels, and the cells are transmitted over an asynchronous transfer mode (ATM) channel. Cells received from the asysnchronous transfer mode (ATM) channel are reassembled into packets concurrently for the plurality of channels.
Accordingly, there is a need for new networks which satisfy the emerging new requirements and which provide broadband circuit switching, fast packet switching, and intelligent network attachments.